As a switching power supply circuit capable of a high speed action, a proposal has been made for the one configured to perform PFM control in a synchronous rectification mode for alternately turning on and off a main switching element and a subordinate switching element by the output of a flip-flop circuit, as shown in FIG. 7. As shown in this drawing, this switching power supply circuit is equipped with a comparator 1 which compares a voltage based on an output voltage VOUT (i.e., a voltage obtained by dividing the output voltage VOUT by a resistance ratio between the resistances of feedback resistors RFB1 and RFB2) with a reference voltage Vref; a flip-flop circuit 2 which is set by the output of the comparator 1; and an ON-time generation circuit 3 which resets the flip-flop circuit 2 at a time when a predetermined ON-time elapses after an output signal from the flip-flop circuit 2 has fallen. The output signal of the flip-flop circuit 2 is supplied to a main switching element (in the present example, a P-channel MOSFET) SW1 or a subordinate switching element (in the present example, an N-channel MOSFET) SW2 via a buffer circuit 4, whereby the main switching element SW1 or the subordinate switching element SW2 is alternately turned on and off to carry out synchronous rectification. In this manner, the output voltage VOUT is obtained which is a predetermined direct current voltage smoothed by a capacitor CL via a coil L connected between a drain of the main switching element SW1 and a drain of the subordinate switching element SW2.
In the above-mentioned switching power supply circuit, when a pulse signal supplied from an output terminal Q_B of the flip-flop circuit 2 to the main switching element SW1 and the subordinate switching element SW2 via the buffer circuit 4 is at an L-level, the main switching element SW1 is in the ON-state and the subordinate switching element SW2 is in the OFF-state. Thus, a coil current ILx flowing through the coil L gradually increases.
When, in this state, the ON-time defined by the ON-time generation circuit 3 has passed, the flip-flop circuit 2 is reset by a reset signal outputted from the ON-time generation circuit 3. As a result, the pulse signal supplied from the output terminal Q_B to the main switching element SW1 and the subordinate switching element SW2 via the buffer circuit 4 is at an H-level, and the main switching element SW1 is in the OFF-state and the subordinate switching element SW2 is in the ON-state. Consequently, the coil current ILx gradually decreases.
As the coil current ILx decreases, the output voltage VOUT becomes less than the reference voltage Vref. At this time, the flip-flop circuit 2 is set, with the result that the main switching element SW1 enters the ON-state and the subordinate switching element SW2 enters the OFF-state. Consequently, the coil current ILx gradually increases. Simultaneously, the management or control of the ON-time by the ON-time generation circuit 3 is started. Afterwards, the same actions are repeated.
In FIG. 7, VIN denotes an input voltage, and CFB denotes a speed-up capacitor. As a prior art document which discloses a switching power supply circuit similar to that in FIG. 7, Patent Document 1 is present.